When motor and generator coils are made, insulated wires are wound or laid to form coils which are impregnated with a resin in a process known as vacuum pressure impregnation (VPI). The VPI resin fills the interstices of the coil, supplementing the insulation on the wires, adding to the mechanical strength of the coil, and preventing the individual wires from rubbing against each other and abrading the insulation.
Until now, various polyester and epoxy resins were used for this purpose. While these VPI resins perform satisfactorily, several of their properties are not as good as is desired. For example, a higher thermal stability is desirable so that the resin does not break down electrically or mechanically at the high temperatures at which the motors and generators operate. It is also desirable to have better high temperature electrical properties such as dielectric constant and dissipation factor. Finally, faster gelling VPI resins are desirable so that the resin does not run out of the interstices of the coil before it gels.